1. Field of the Invention
The present invention relates generally to optical fibers, and more specifically to methods for the fabrication of optical fibers and optical fiber preforms.
2. Technical Background
Optical fibers formed completely from glass materials have been in commercial use for more than two decades. Although such optical fibers have represented a leap forward in the field of telecommunications, work on alternative optical fiber designs continues. One promising type of alternative optical fiber is a microstructured optical fiber, which includes holes or voids running longitudinally along the fiber axis. The holes generally contain air or an inert gas, but may also contain other materials.
Microstructured optical fibers may be designed to have a wide variety of properties, and may be used in a wide variety of applications. For example, microstructured optical fibers having a solid glass core and a plurality of holes disposed in the cladding region around the core have been constructed. The arrangement, spacings and sizes of the holes may be designed to yield microstructured optical fibers with dispersions ranging anywhere from large negative values to large positive values. Such fibers may be useful, for example, in dispersion compensation. Solid-core microstructured optical fibers may also be designed to be single mode over a wide range of wavelengths. Solid-core microstructured optical fibers generally guide light by a total internal reflection mechanism; the low index of the holes can be thought of as lowering the effective index of the cladding region in which they are disposed.
One especially interesting type of microstructured optical fiber is the photonic band gap fiber. Photonic band gap fibers guide light by a mechanism that is fundamentally different from the total internal reflection mechanism. Photonic band gap fibers have a photonic crystal structure formed in the cladding of the fiber. The photonic crystal structure is a periodic array of holes having a spacing on the order of the wavelength of light. The core of the fiber is formed by a defect in the photonic crystal structure cladding. For example, the defect may be a hole of a substantially different size and/or shape than the holes of the photonic crystal structure. The photonic crystal structure has a range of frequencies, known as the band gap, for which light is forbidden to propagate in the photonic crystal structure. Light introduced into the core of the fiber having a frequency within the band gap will not propagate in the photonic crystal cladding, and will therefore be confined to the core. A photonic band gap fiber may have a core that is formed from a hole larger than those of the surrounding photonic crystal structure; in such a hollow-core fiber, the light may be guided in a gaseous medium, lowering losses due to absorption and Rayleigh scattering of glass materials. As the light is guided in a gaseous medium, hollow-core fiber may also have extremely low non-linearity.
Microstructured optical fibers are fabricated using methods roughly analogous to the manufacture of all-glass optical fiber. A structured preform having the desired arrangement of holes is formed, then drawn into fiber using heat and tension. In one conventional method of making a structured optical fiber preform, hexagonal-sided hollow glass tubes are stacked together to form an assembly having a lattice structure, and one or more of the tubes are removed to form a core volume in the center of the lattice structure. Since the core volume is formed by the removal of one or more tubes from the assembly, it has a shape and size restricted to integer multiples of the shape of the hexagonal-sided tubes of the assembly. The lattice structure is redrawn to reduce its cross-sectional size, then sleeved and drawn into optical fiber having a lattice-like array of holes surrounding a core defect void. The core defect void has a shape and size corresponding to the shape and size of the core volume. Since the optical properties (e.g. dispersion, attenuation) of the fiber may depend strongly on the cross-sectional size and shape of the core defect void, a method for making a microstructured optical fiber having a core defect void with an arbitrary shape and size would be advantageous.